I’m glad I saw this post before it happened to me: Kalpa Desktop: "Heads up, #Kalpa users I made a change in plasma…" - Fosstodon
In short, after log in (e.g. via tty) run:
sudo systemctl disable display-manager-legacy.service
sudo systemctl enable --now sddm.service
and the GUI should come up.
Although my old MicroOS Desktop KDE (alpha) install is not branded as Kalpa, it often seems like a good idea to follow Kalpa for anything KDE related.